In pervaporation, a multi-component liquid stream is passed across a membrane that preferentially permeates one or more of the components. As the feed liquid flows across the membrane surface, the preferentially permeated components pass through the membrane and are removed as a permeate vapor.
Transport through the membrane is induced by maintaining a vapor pressure on the permeate side of the membrane that is lower than the vapor pressure of the feed liquid. The vapor pressure difference is usually achieved by maintaining the feed liquid at a higher temperature than that of the permeate stream. The latent heat of evaporation of the permeate components must be supplied to the feed liquid for the feed temperature to be maintained and for the pervaporation process to continue.
In a typical separation, such as that of an alcohol from water, the feed cools about 5° C. for every 1% of the feed that permeates the membrane. In industrial pervaporation processes, an average of about 5% of the feed permeates the membrane per module. The corresponding temperature drop is thus about 25° C. Other separations may involve greater or lesser temperature drops.
The temperature drop is reversed by withdrawing the feed solution and running it through individual heaters or heat exchange cycles between each module. This is commonly referred to as inter-stage heating.
To accommodate these requirements for inter-module or inter-stage reheating, pervaporation systems must include numerous pipes, flanges, valves and other fittings to enable the feed solution to pass in and out of the vessel housing the modules. This makes the systems complex to engineer, cumbersome to build, and costly, and limits the industrial utility of pervaporation.
In consequence, although commercial pervaporation systems have been available for more than twenty years, few practical applications for a process that is otherwise attractive have been realized.
Co-owned and co-pending patent application Ser. Nos. 11/050,995; 11/271,402; and 11/484,547 disclose gas separation equipment in which multiple membrane modules in multiple tubes are contained in a single housing. These three patent applications are incorporated herein by reference in their entirety.